Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
Influence of the Processes Parameters on the Properties of The ...
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Chapter 2.<br />
<str<strong>on</strong>g>Processes</str<strong>on</strong>g> to Manufacture Foams and to Functi<strong>on</strong>alize <str<strong>on</strong>g>the</str<strong>on</strong>g> Surface<br />
<br />
<br />
mechanical properties matching those <str<strong>on</strong>g>of</str<strong>on</strong>g> tissues at <str<strong>on</strong>g>the</str<strong>on</strong>g> site <str<strong>on</strong>g>of</str<strong>on</strong>g> implantati<strong>on</strong>,<br />
easily processable to form a variety <str<strong>on</strong>g>of</str<strong>on</strong>g> shapes and sizes.<br />
In batch foaming, a polymer in disc or powder form is subjected to supercritical CO 2 flow without<br />
mixing. After venting <str<strong>on</strong>g>the</str<strong>on</strong>g> CO 2 by depressurizati<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g>rmodynamic instability causes supersaturati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
CO 2 dissolved in <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer matrix and hence, nucleati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> cells occurs. <strong>The</strong> growth <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> cells c<strong>on</strong>tinues<br />
until <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer vitrifies. <strong>The</strong> saturati<strong>on</strong> pressure, <str<strong>on</strong>g>the</str<strong>on</strong>g> saturati<strong>on</strong> temperature and <str<strong>on</strong>g>the</str<strong>on</strong>g> depressurizati<strong>on</strong> rate<br />
are <str<strong>on</strong>g>the</str<strong>on</strong>g> critical parameters in determining <str<strong>on</strong>g>the</str<strong>on</strong>g> number <str<strong>on</strong>g>of</str<strong>on</strong>g> cells and <str<strong>on</strong>g>the</str<strong>on</strong>g> cell size distributi<strong>on</strong>. To predict <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
pore size <str<strong>on</strong>g>of</str<strong>on</strong>g> foams created, in <str<strong>on</strong>g>the</str<strong>on</strong>g> literature, <str<strong>on</strong>g>the</str<strong>on</strong>g>re is several numbers <str<strong>on</strong>g>of</str<strong>on</strong>g> influential studies <strong>on</strong> diffusi<strong>on</strong>induced<br />
pore growth. <strong>The</strong>se models c<strong>on</strong>sider <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer as a Maxwell fluid and solve systems <str<strong>on</strong>g>of</str<strong>on</strong>g> partial<br />
differential equati<strong>on</strong>s by numerical methods [Goel and Beckman, 1995; Arefmanesh and Advani, 1991].<br />
<strong>The</strong> cell number density increased and <str<strong>on</strong>g>the</str<strong>on</strong>g> cell size decreased with increasing pressure and<br />
decreasing temperature. A high degree <str<strong>on</strong>g>of</str<strong>on</strong>g> super-saturati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> dissolved CO 2 at high pressure and low<br />
temperature are resp<strong>on</strong>sible for such results. Classical homogeneous nucleati<strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g>ory is generally used to<br />
calculate <str<strong>on</strong>g>the</str<strong>on</strong>g> nucleati<strong>on</strong> rate in foaming with supercritical CO 2 . <strong>The</strong> energy barrier for nucleati<strong>on</strong> in <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g>ory can be calculated as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> interfacial tensi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> binary mixture and <str<strong>on</strong>g>the</str<strong>on</strong>g> magnitude <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop. <strong>The</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g>ory suggests that <str<strong>on</strong>g>the</str<strong>on</strong>g> energy barrier and <str<strong>on</strong>g>the</str<strong>on</strong>g> interfacial tensi<strong>on</strong> decrease as <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
pressure drop increases. C<strong>on</strong>sequently, <str<strong>on</strong>g>the</str<strong>on</strong>g> nucleati<strong>on</strong> rate increases and a large number <str<strong>on</strong>g>of</str<strong>on</strong>g> small cells is<br />
obtained. In fact, both <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop rate and <str<strong>on</strong>g>the</str<strong>on</strong>g> magnitude <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop determine <str<strong>on</strong>g>the</str<strong>on</strong>g> cell<br />
density and cell size in microcellular foaming. <strong>The</strong> higher <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop rate, <str<strong>on</strong>g>the</str<strong>on</strong>g> greater <str<strong>on</strong>g>the</str<strong>on</strong>g> nucleati<strong>on</strong><br />
rate due to <str<strong>on</strong>g>the</str<strong>on</strong>g> high supersaturati<strong>on</strong> rate. This allows <strong>on</strong>ly a short time for existing cells to grow and,<br />
c<strong>on</strong>sequently, is in favour <str<strong>on</strong>g>of</str<strong>on</strong>g> formati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> small cells. Classical nucleati<strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g>ory fails to incorporate <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
effect <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> pressure drop rate. Moreover, a noteworthy study <strong>on</strong> CO 2 -assisted microcellular foaming <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
PLGA is reported by Sparacio and Beckman [1998], in which a minimum in cell size with increasing<br />
pressure was found instead <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> levelling <str<strong>on</strong>g>of</str<strong>on</strong>g>f according to <str<strong>on</strong>g>the</str<strong>on</strong>g>ory. A plausible explanati<strong>on</strong> is low resistance<br />
to cell growth due to a large decrease in <str<strong>on</strong>g>the</str<strong>on</strong>g> melting point <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer and very low interfacial tensi<strong>on</strong> at<br />
high pressure.<br />
Detailed studies <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> glassy polymer – CO 2 system by Wessling et al. [1994] suggest that <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
nucleati<strong>on</strong> mechanism underlying <str<strong>on</strong>g>the</str<strong>on</strong>g> foaming process is heterogeneous in nature. <strong>The</strong> significant advance<br />
made by [Wessling et al., 1994] was that <str<strong>on</strong>g>the</str<strong>on</strong>g>y were able to detect and explained <str<strong>on</strong>g>the</str<strong>on</strong>g> appearance not <strong>on</strong>ly <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> porous structure in <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer film after saturati<strong>on</strong> with CO 2 but also <str<strong>on</strong>g>of</str<strong>on</strong>g> a dense layer next to <str<strong>on</strong>g>the</str<strong>on</strong>g> porous<br />
layer. <strong>The</strong>y provided a physical explanati<strong>on</strong> and a ma<str<strong>on</strong>g>the</str<strong>on</strong>g>matical model to predict <str<strong>on</strong>g>the</str<strong>on</strong>g> thickness <str<strong>on</strong>g>of</str<strong>on</strong>g> this dense<br />
layer. <strong>The</strong> studies <str<strong>on</strong>g>of</str<strong>on</strong>g> McCarthy and coworkers [Stafford et al., 1999; Arora et al., 1998a] <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> effect <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
residual oligomer in polystyrene <strong>on</strong> its foaming with scCO 2 have shown that its presence affects <str<strong>on</strong>g>the</str<strong>on</strong>g> cell size<br />
in <str<strong>on</strong>g>the</str<strong>on</strong>g>se foams. This work also questi<strong>on</strong>ed <str<strong>on</strong>g>the</str<strong>on</strong>g> ability <str<strong>on</strong>g>of</str<strong>on</strong>g> classical nucleati<strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g>ory to explain <str<strong>on</strong>g>the</str<strong>on</strong>g> foaming<br />
mechanism in <str<strong>on</strong>g>the</str<strong>on</strong>g>se systems, and <str<strong>on</strong>g>the</str<strong>on</strong>g> authors suggest a spinodal mechanism as an alternative route <str<strong>on</strong>g>of</str<strong>on</strong>g> cell<br />
formati<strong>on</strong> [Stafford et al., 1999]. Foaming <str<strong>on</strong>g>of</str<strong>on</strong>g> polypropylene has also been studied extensively by Park and<br />
Cheung [1997], with <str<strong>on</strong>g>the</str<strong>on</strong>g> most recent report by Liang and Wang [1999], who highlighted <str<strong>on</strong>g>the</str<strong>on</strong>g> effect <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
temperature drop during depressurizati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer in equilibrium with high-pressure CO 2 . Handa and<br />
Zhang [2000] used <str<strong>on</strong>g>the</str<strong>on</strong>g> existence <str<strong>on</strong>g>of</str<strong>on</strong>g> a rubbery state in <str<strong>on</strong>g>the</str<strong>on</strong>g> PMMA at low temperatures to generate foams by<br />
saturating <str<strong>on</strong>g>the</str<strong>on</strong>g> polymer with CO 2 at 24C to 90C. <strong>The</strong>y dem<strong>on</strong>strated that <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 in <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
polymer plays an important role in c<strong>on</strong>trolling cell density and cell size. Thus, <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility <str<strong>on</strong>g>of</str<strong>on</strong>g> CO 2 at 34<br />
bars and temperature <str<strong>on</strong>g>of</str<strong>on</strong>g> – 0.2C is 22.5% (w/w), while at <str<strong>on</strong>g>the</str<strong>on</strong>g> same pressure but at 24C, <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility is just<br />
7.9% (w/w).<br />
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